Code B121, Twilight Enable Switch Circuit: Notes

The twilight enable switch uses the same physical control as delay pot. Inside left switch assembly, enable switch contacts close as soon as time delay slider is moved off of its extreme minimum (off) position. The BCM supplies a voltage on circuit No. 304 to switch. When slider is moved from off position, switch contacts close and signal voltage is pulled low through ground circuit No. 804.

Code B121 will set if vehicle is equipped with twilight sentinel, twilight delay pot has not failed (Code B120), and signal voltage is high when delay slider is greater than 2 seconds (twilight sentinel on), or signal voltage is low when delay slider is less than 3 seconds (twilight sentinel off). During time failure is present, twilight sentinel will operate based on delay input. Whenever delay is greater than 2 seconds, BCM will assume twilight sentinel is on.

1. BI82 displays voltage state of circuit at BCM. These conditions can be observed in BI82 display as readings of HI or LO when slider is moved.
2. Checks to see if HI reading is due to circuit or switch assembly. If display changes from HI to LO after jumping switch assembly terminals, BCM and wiring are okay.
3. Checks to see if LO reading is due to circuit or switch assembly. If display changes from LO to HI when switch assembly is disconnected, both BCM and wiring are okay.